As a technique for supporting layout and the like of base stations when building a radio communication system, a radio wave propagation characteristic estimation system (a radio wave propagation simulator) is known. This radio wave propagation simulator is a system which sets location data of a transmission point that is a point from which a radio wave is emitted and location data of a reception point that is a point at which the radio wave is received, and calculates a radio wave state at the reception point. By evaluating received power and delay expansion at an arbitrary reception point calculated by this system, the base station set place, parameters, or the like can be determined. This makes it possible to efficiently secure the areal communicable area, to reduce the number of base stations to be located, etc.
Known radio wave propagation simulators can be roughly classified into those using a statistic method and those using a deterministic method. The statistic method is a method which gives a propagation loss estimation formula using a distance, frequency, and the like as arguments, and uses a multivariate analysis and the like based on a great amount of data acquired by actual measurements of propagation loss when determining the parameters.
A radio wave propagates while being reflected, transmitting, and diffracted at undulations and structures such as buildings (referred to as objects hereinafter). However, the statistic method estimates only the attenuation amount of radio waves according to the distance from the base station, direction, and the frequency of the transmission radio wave regardless of objects in the surroundings of the reception point. Thus, it is not possible to estimate the local radio wave propagation environment highly accurately.
In the meantime, the deterministic method is a method which estimates the radio wave propagation based on influences imposed upon the radio wave caused by the objects. As one of such deterministic methods, there is a ray tracing method. The ray tracing method is a method which considers radio waves emitted from an antenna of a radio wave dispatching source as a group of a great number of radio wave rays, traces each ray as a wave that propagates geometrical-optic manner while repeating reflection and transmission, and acquires the loss and delay amount of the radio waves at an observation point by synthesizing a great number of rays arriving at the observation point. The ray tracing method estimates the radio wave propagation by adding the influences of reflection, transmission, and diffraction at the objects, so that it is possible to perform highly accurate estimation of the radio wave propagation.
Particularly, when an upper-floor of a high-rise building is set as an observation point, it is likely to have issues of deterioration in the radio wave quality due to interference and failures in handover because an unobstructed view from the surroundings can be secured so that radio waves from a great number of base stations located at distant places can reach thereto. In order to investigate proper measures by grasping the radio wave conditions inside such building without making a great effort, the radio wave propagation simulation is effective. Under an environment such as an upper-floor of a high-rise building, it is effective to use the ray tracing method which takes the reflection and diffraction of the radio waves at the surrounding objects into consideration.
However, with the deterministic method, an arithmetic calculation processing amount becomes increased since the reflection points and the diffraction points of the radio waves are increased when there are a great number of objects within an analysis target area. Specifically, the arithmetic calculation processing amount is increased roughly in proportional to a square of the number of objects. As a result, a great amount of time is required for the radio wave propagation estimation.
In order to overcome such issues, Patent Documents 1 and 2 disclose techniques which speed up the arithmetic calculation processing of the radio wave propagation estimation with the ray tracing method.
Patent Document 1 discloses a technique which limits the path of the rays by searching the paths of the roads from a transmission point to a reception point and intersection points under a street micro cell environment where base stations are located at locations lower than surrounding buildings, and performs estimation of the radio wave propagation condition by excluding objects located at locations distant from the searched paths. Under the street micro cell environment, there is only a small influence on the accuracy of the radio wave propagation estimation even if the buildings distant from the areas along the roads are ignored since the radio waves mostly propagate along the roads. Thus, it is possible with the technique disclosed in Patent Document 1 to speed up the arithmetic calculation processing without largely deteriorating the accuracy.
Patent Document 2 discloses a technique which refers to data of buildings within a specified area based on the distance from a transmission point and a reception point, and traces the track of the radio waves propagated from the transmission point to the reception point. It is possible with Patent Document 2 to estimate the radio wave propagation characteristic from the transmission point to the reception point at a high speed without deteriorating the accuracy.    Patent Document 1: Japanese Patent No. 3092651    Patent Document 2: Japanese Unexamined Patent Publication 2003-318811